As noted in the article entitled "Plaster Molding For Precision Castings" by Dave Kanicki, which appeared in MODERN CASTING, MAX 1978, the plaster mold process is a quick and economical means for the production of prototype castings in small volume. In the manufacture and testing of new machinery designs, the process is valuable in that experimental devices can be produced and tested while avoiding the massive tooling costs which would otherwise be encountered if other production routes such as die-casting were resorted to so that the required parts could be obtained. The process works well for the production of castings made of aluminum and zinc and their alloys. However, in the case of magnesium and magnesium alloys, the reactivity of the metal is so great that explosions result when it is attempted to produce castings thereof in the kinds of plaster molds which can successfully be employed to produce aluminum and zinc castings.
U.S. Pat. Nos. 2,529,835 and 2,754,220 are directed to the production of permeable plaster of Paris molds wherein gypsum (CaSO.sub.4.sup.. 1/2 H.sub.2 O) plaster is mixed with foam and other additives to produce a fine pore structure within the molds which may then be heated to the extent of calcination of the calcium sulfate hemihydrate. Such a calcination procedure is not viewed favorably in the patents. The use of sulfur hexafluoride (SF.sub.6) as a mold purge in the production of magnesium castings has been known.